1. Field of the Invention
This invention relates to an electric motor provided with a split stator core including a plurality of circumferentially disposed unit cores and a method of making such a motor.
2. Description of the Prior Art
For the purpose of effective utilization of steel material, the prior art has provided an annular split stator core formed by disposing a plurality of circumferentially split unit cores into a generally circularly or squarely annular configuration. More specifically, when annular steel sheets which are to be stacked into a stator core are punched out of steel sheets, portions of each steel sheet outside and inside the annular configuration are left unused. The above-mentioned annular split stator core provided by the prior art is directed to a reduction in such unused portions of the steel sheets.
However, a location of portions of the unit cores adjacent to each other is selected at random. This results in unbalance in magnetic attractive forces acting between the unit cores, whereupon vibration and noise are produced.
Each of a number of steel sheets stacked together into a unit core is formed by punching a silicon steel sheet having a surface treated for electrical insulation by a press. The punching sometimes results in warpage and/or burrs in ends of the silicon steel sheet. In a stator core formed by annularly disposing a plurality of unit cores, when the unit cores adjacent to each other are displaced in the direction of stack of the steel sheets or when one or more steel sheets have the warpage and/or burrs, the steel sheets of each unit core are electrically short-circuited by the ends of the steel sheets of the other unit core. This results in eddy currents flowing in the direction of stack of steel sheets in the unit core, so that an iron loss is increased.